One of the routine steps in the production process of integrated circuits is the inspection of patterned surfaces, especially when starting up a new design. A substantial part of the whole 300 mm wafer is imaged to check for defects in the pattern and for particles imbedded in the pattern or on top of the wafer. This kind of inspection is presently performed by high-throughput optical microscopy in dedicated instruments.
With the progress in lithography, it is desirable that instruments detect smaller and smaller defects and particles. A problem is that the light scattering from particles rapidly decreases when the particle's size decreases, so the signal-to-background (and noise) ratio is decreasing.
In order to solve this problem, electron beam inspection machines have been used and for some purposes are still in use. Electron beam inspection machines can have a much higher resolution than optical system. However, electron beam inspection machines are limited in the speed at which the electron beam inspection machines can inspect a wafer. In order to increase the speed, multi-beam electron beam systems have been proposed.
US 2007/0272856, described a method and an apparatus for inspecting a specimen surface. The method comprises the steps of generating a plurality of primary beams directed towards the specimen surface, focusing the plurality of primary beams onto respective loci on the specimen surface, collecting a plurality of secondary beams of charged particles originating from the specimen surface upon incidence of the primary beams, converting at least one of the collected secondary beams into an optical beam, and detecting the optical beam. The apparatus described in this Patent Publication comprises a screen with fluorescent material, which screen is disposed between an emitter for generating a plurality of primary beams and the specimen surface, the primary beams are focused at the level of the screen and which screen is constructed such that the primary beams can traverse through holes in the screen. The secondary beams from the specimen surface are directed from the specimen surface towards the screen and are defocused on the screen to provide a spot which covers an area around the holes in the screen. At these spots, the secondary beams are converted into optical beams, which optical beams are then imaged through free space onto a plane in which the optical detector is located.